JavaScript - random multiple of 20 between 0 - 580

Question

In Javascript, how do I create a random even number multiplied by 20 between 0 - 580?

E.g.: 220, 360, 180, 0 (min), 400, 200, 580 (max)

Answer 1

You want increments of 20, so what you really need is an integer in the range 0 to 29, and then multiply with 20. Example:

var max = (580/20) + 1;
var result = 20 * (Math.floor(Math.random())*max)

We are adding one to max, because Math.random() is a uniformly distributed number between (inclusive 0) and (exclusive 1), so since we use Math.floor, the maximum must be 1 larger.

Answer 2

This way creates a random number, then rounds it down to the nearest multiple:

When you need 0 <= randomMultiple <= max

var random = Math.random() * (580 + 20);
randomMultiple = random - (random % 20);

When you need 0 <= randomMultiple < max

var random = Math.random() * 580;
randomMultiple = random - (random % 20);

Answer 3

Use a principle like this: Generate random number between two numbers in JavaScript

Keeping in mind that if you want your max result to be 580, then the maximum integer you want to multiply by 20 would be 29 (or 580/20). Then just add some logic to make sure the integer is even.

Ta da!

Answer 4

Try use this:

var result = parseInt(Math.random()*30)*20;

29*20 = 580

Math.random() return [0..1)

result between 0..580, step by 20

Answer 5

Here is a generic javascript one-liner that can be used for any range, and any multiple.

Essentially, what we are trying to do here is figure out a range, 0 to N, which when multiplied by our given multiple stays within the range [0,max-min].

In this case N, is simply, (max - min)/multiple, or range/multiple.

Once we have N, we can use Math.random() to get a random number between 0-N, and multiply it with multiple. Next, we just add min.

We assume that min and max are already multiples of multiple.

Note the additional +1 to the input of Math.random() is because Math.random() returns a number between 0 (inclusive) and 1 (exclusive). So, Math.random() can never return 1. If we didn't account for that we would never be able to include the max number in our results.

/*
 * Returns a random number within range [min,max]
 *
 * min and max must be multiples of multiple
 * (note that 0 is a multiple of all integers)
 */
function randomMultiple (min, max, multiple) {
    return Math.floor(Math.random() * (((max - min)/multiple)+1)) * multiple + min;
}

console.log(randomMultiple(0, 580, 20));